I don't know about actual dimensions but if the tube was less than 3" it shouldn't make much difference. The perch just welds to a small portion of the total circumference. Even with a 2 1/2" tube the gap should be 1/8" or less on each side. Easy to fill with an arc.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.